Focusing on the process of grammaticization, whereby items with lexical meaning evolve into grammatical markers, this article examines the future temporal reference sectors of three diaspora varieties of African American English which have evolved in linguistic isolates and compares them with those of British-origin rural and mainstream varieties of English. With one exception, the same constraint hierarchies condition the selection of going to across the board, indicating that their future temporal reference systems are descended from a common source. All other distinctions among the varieties result from their differential positioning on the cline of ongoing grammaticization of going to as a future marker. Operationalization of constraints representing different stages of the development of going to and comparison of their probability values across communities confirm that the enclave and the rural varieties retain conservative traits, visible here in the form of variable conditioning, in contrast to mainstream English, which is innovating. We suggest that the major determinant of variability in the expression of the future is the fact that the speech of isolated speakers, whether of African or British origin, instantiates constraints that were operative at an earlier stage of the English language and that are now receding from mainstream varieties.